Ultrahigh frequency oscillator



April 5, 1938. J. EVANS ULTRAHIGH FREQUENCY OSCILLATOR Filed Sept. 21,1935 IIII IN vB/v'roR John Evans Witness.- 00 '7 HT'I'OHNEY PatentedApr. 5, 1938 UNITED STATES PATENT OFFICE John Evans, Collingswood, N. Jassignor to Radio Corporation of America, a corporation of DelawareApplication September 21, 1935, Serial No. 41,540

8 Claims.

My invention relates to ultrahigh frequency thermionic devices. Moreparticularly, my invention is an ultrahigh frequency oscillatoremploying thermionic tubes in which the anodes are a virtually at groundpotential.

I am aware of numerous circuits for generating high frequencyoscillations. Some of the proposed circuits will generate ultrahighfrequencies but the frequency is far from constant. Other circuits havebeen devised which generate oscillations of high and constant frequencybut it is impossible to extend the range to frequencies exceeding fiftymegacycles. Beyond these high frequencies the thermionic tube elementsand the leads within the tubes assume electrical lengths of proportionswhich interfere with the higher frequency ranges.

One of the objects of my invention is the generation of oscillatorycurrents of ultrahigh frequencies.

Another object of my invention is to reduce the effect of the anode loadon the input circuits of a thermionic tube.

A further object is to place the anodes of a pair of push-pulloscillator tubes at virtually ground potential.

Additional objects will appear in the following specification andclaims.

Figure 1 is a schematic diagram of an ultrahigh frequency oscillator inwhich the anodes are at ground potential, and

Figure 2 is a diagram of an oscillator embodying one form of myinvention.

It is well known to those skilled in the art that the characteristics ofthe input circuit of a 'triode are dependent on the load in its outputcircuit. Although this efiect may not be of great importance at lowfrequencies, it is of extreme importance at high frequencies.

ample, with one type of tube which has an input capacity of about 28micromicrofarads with a zero plate circuit load, the effective inputcapacity will be approximately three times as great with an externalload of 100,000 ohms. The output load may be inductive, capacitive orresistive and in each case the effective capacity or reactance of theinput circuit will be effected.

The input reactance may be efiected in either a positive or negativesense. 7

Although these effects are not always deleterious, I have found that atultrahigh frequences the plate circuit load will limit the range ofoscillation and ultimately limit the highest oscillatory frequency whichcan be reached. At frequencies of the order of. megacycles and lower,

By way of ex-- depending mainly on the tube construction, the circuit ofFigure 1 may be used.

In Figure 1 a pair of triode tubes l-3 are connected in push-pullrelation. The grids 5-l are connected to the conductors 9ll of atransmission line l3. A bridging member I5 is adjusted on the 'lines 9-Huntil a quarter wave length characteristic is obtained. The center I! ofthe bridge is grounded through a grid leak resistor Hi. The cathodes2l-23 may be energized by any suitable source; for example batteries25-21 or alternating current. In practice the capacity of the batteriesor other source of cathode heating energy must be considered. Eitherfilter networks or concentric lines should be employed between thebatteries and the cathodes as will be described below. The batteries25-21 in Fig. 1 are shown as directly connected to the cathodes 2|, 23merely for convenience of illustration.

Conductors 293l of a second transmission line 33 are connected to thecathodes 2 l-23. An adjustable bridging member 35 is connected to thetransmission line at about a quarter wave length position at which thetuned grid, tuned cathode will cause oscillations. The center 3! of thecathode bridging member is grounded. The anodes 394l are connectedtogether with the shortest. possible leads. The center 43 of the leadsbetween anodes is connected to ground through a suitable capacitor 45.The center is also connected to a radio frequency choke coil 41. Thechoke in turn is connected to the positive terminal of the anode currentsource 49. The negative terminal of the anode current supply 49 isgrounded.

In the arrangement shown in Figure 1, if the length of the leads betweenthe anodes is very short, the anodeswill be maintained at groundpotential for radio frequency currents because of the low reactance ofthe capacitor 45 which virtually grounds the anodes. Under favorablecircumstances the anode output circuit will have substantially zeroimpedance and the effective input capacity will be relatively low andpermit the generation of oscillatory currents of a frequency of theorder of 50 megacycles. However, as the frequency is graduallyincreased, the leads between the anodes, and particularly the leadswithin the tube envelopes, will assume sub- 0 The proper length of thetransmission put circuit and effect the characteristics of the inputcircuit as explained above.

I propose to overcome this difliculty by arranging an output circuitwhich will have substantially no effect on the oscillation frequency andwhich will place the anodes at virtually ground potential at the highestfrequencies. In

Figure 2 a diagram of a circuit employing anodes at virtual groundpotential is shown. A pair of triode thermionic tubes 5 1-52 areconnected as a push-pull oscillator. The grids 53-55 are connectedrespectively to the conductors 5'l--59 of a transmission line 6|. Theline is adjustedto a quarter wave length by moving a bridging member 63to the required position. The center 65 of the bridging member isconnected to ground through the grid leak resistor 61.

The cathodes 69--'H are each energized by batteries 'l3l5 or othersuitable source. Radio frequency choke coils Ill, 12 are connected "in"each of the leadsbetween the batteries 13, 15 and the cathodes 69- 11.The batteries are eachgrounded. Ag-pairof capacitors 14 and 16 areconnected in series across each of the cathode leads. The junction ofone pair of capacitors 14,

i4 is connected to one of the conductors ll of a transmission ,-line 8|;pair of capacitors I6, 16 is connected to the other conductor of thetransmission line. Although I have shown the filter network between thebatteries and the cathodes, .it is equally effective to make thetransmission line 8| a pair of hollow conductors. One wire is connectedwithin each conductor from the battery to the cathode and the hollowconductors are used for the return wires from each cathode to thebattery. The cathode leads are by-passed by a suitablecapacitor. Ineither type of connection, it is preferable to make the transmissionlines straight, parallel and of equal length.

line to sustain oscillations is found by moving the bridging member 83whose center point 85 is grounded. The anodes 8'l--89 are connected tothe conductors 9l93 of a transmission line 95. Across the end of thetransmission line opposite the anodes is placed the bridging member 91.The center 99 of the bridging member 91 is grounded through a capacitorlfll whose reactance is low to the currents generated b-y the push-pulloscillator.

The center. 99 of the bridging member is also connected through'a radiofrequency choke I03 to the positive terminal of the anode currentsupply. N15. The negative terminal of this supply is grounded.

If the bridging member 91 is carefully adjusted so that the transmissionline is effectively a half wave length long, the anodes will bevirtually at ground potential for the highfrequency currents. The effectof the virtually grounded anodes is to make the output load of the anodecircuit substantially zero and to remove the effects of this circuit onthe input circuit. Using RCA type .852 tubes in push-pull relation withtuned input and tuned cathode-circuits, I have been .able to gener- Thejunction of the other 1. In an ultrahigh frequency oscillator of thethermionic tube type in which the inductance of the anode electrode leadhas sufficient reactance at said ultrahigh frequency to preventgrounding the anode electrode within the tube, a pair of thermionictubes, each of said tubes including grid. cathode and anode electrodes,2. ground connection, a grid circuit connected between saidgridelectrodes and said ground, a cathode circuit connected between'saidcathode electrodes and said ground, means for tuning said grid andcathode circuits with reference to the frequency of the oscillations tobe produced, an anode circuit connected to said anode electrodes, meansfor grounding said anode circuit, and

means for tuning said anode circuit to effectively a half wave length ofsaid ultrahigh frequency oscillation wherebysaid grid and cathodecircuits determine the oscillatory frequency of said -'oscillator andsaid anode circuit establishes an ultrahigh frequency groundhpotentialon said anode. electrodeswithin said envelope.

2..In an, ultrahigh frequency oscillator of the thermionic ,tube type inwhich the anode lead offers a reactance at the frequency of operationwhich prevents directly groundingthe anode electrode within said tube, apairof thermionic tubes, including, within. an evacuated envelope grid,cathode and anode electrodes, arranged to operate on opposite phases ofoscillatory currents; a ground connection, a grid circuit connectedbetween said grid electrodes and'ground, a cathode circuit connectedbetween saidcathode electrodes and ground, means for tuning said gridand cath- OdQ'CiICUItS to said frequency of operation, means forimpressing a positive direct current potential with respect to saidcathodes on said anodes, and means for establishing, an ultrahighfrequency ground potential on said anodes within said envelope.

3. In an ultrahigh frequency oscillator of the type, in which the anodelead of a thermionic tube offers sufficient reactance to the frequencyof operation to prevent directly grounding, the

anode within said tube, a pair of thermionic tubes including within anevacuated envelope grid,

' cathode and anode electrodes and arranged to operate on oppositephases of their generated currents, a ground connection, a grid circuitconnected between said-grid electrodesand ground, a cathode circuitconnected between said cathode electrodes and ground, means for tuningsaid grid and cathode circuits to said-frequency of opera-,-

tion,;means for impressing -a positive direct current potential betweensaid anode and cathode electrodes, and means: for establishing anultrahigh frequency ground potential within said envelope on said anodeswith respect to said ground connections.

4.1m a push-pull ultrahigh frequency thermionic oscillator of the typein which the anode lead offers a reactance at the frequency of operationwhich prevents directly grounding the anode within said tube, a pair ofthermionic tubes having inputfanode and cathode electrodes; a groundconnection, a transmission line of substantially one quarter wave lengthconnected between said input electrodes and ground, a secondtransmission line of substantially a quarter wave length connectedbetween said cathode elec-. trodes and ground, and means .formaintaining said anodes at points within said thermionic tubes at groundpotential with respect to said ground connection for ultrahigh frequencycurrents and at positive direct current potential with respect to saidcathode electrodes.

5. In a push-pull ultrahigh frequency thermionic oscillator of the typein which the anode lead oiiers a reactance at the frequency of operationwhich prevents directly grounding the anode Within said tube, a pair ofthermionic tubes having grid, anode and cathode electrodes, a groundconnection, a transmission line of substantially one quarter wave lengthconnected between said grid electrodes and said ground, a secondtransmission line of substantially a quarter wave length connectedbetween said cathode electrodes and said ground, and a thirdtransmission line of substantially a half wave length connected to saidanode electrodes at one end and to ground at the other end and tunewhereby the first mentioned end of said third transmission line issubstantially at the potential of said ground connection with respect tohigh frequency currents, and a source of anode current supply, having agrounded negative terminal and a positive terminal connected to thesecond mentioned end of said third line.

6. In an ultrahigh frequency thermionic oscillator of the type in whichthe anode lead offers a reactance at the frequency of operation whichprevents directly grounding the anode within said tube, a thermionictube having an evacuated envelope including grid, cathode and anodeelectrodes; a ground connection, a grid circuit connected between saidgrid electrodes and ground, a cathode circuit connected between saidcathode electrodes and ground and an anode circuit connected betweensaid anode electrodes and ground, means for tuning said grid and cathodecircuits to generate oscillations at said operating frequency, means foradjusting the effective electrical length of said anode circuit to ahalf wave length at said operating frequency whereby said anodeelectrode within said envelope will be at ground potential for said highfrequency oscillations, a direct current supply and connectionstherefrom for positively biasing said anode electrode with respecttosaid cathode.

7. In a device of the character described in claim 4, means foradjusting the effective length of said transmission lines.

8. In a device of the character described in claim 5, means foradjusting the effective length of said transmission lines.

JOHN EVANS.

